1. Field of the Invention
The present invention relates to filters and more specifically relates to multilayer LC filters.
2. Description of the Related Art
A multilayer dielectric filter described in Japanese Unexamined Patent Application Publication No. 2006-33614 is an example of a known conventional filter. Hereafter, the multilayer dielectric filter described in Japanese Unexamined Patent Application Publication No. 2006-33614 will be described with reference to the drawings. FIG. 6 is an exploded perspective view of a multilayer dielectric filter 500.
The multilayer dielectric filter 500 includes a multilayer body 502 and outer electrodes 503 (503a to 503d). The multilayer body 502 has a rectangular parallelepiped shape and is formed by stacking rectangular insulator layers 504 (504a to 504j) on top of one another. The outer electrodes 503 are respectively provided on the four side surfaces of the multilayer body 502. The outer electrode 503a is a high-frequency signal input terminal. The outer electrode 503b is a high-frequency signal output terminal. The outer electrodes 503c and 503d are ground terminals.
In addition, resonators 505 (505a to 505c) are built into the multilayer dielectric filter 500. The resonators 505a to 505c respectively include resonance electrodes 506a to 506c. The resonance electrodes 506a to 506c are provided on the insulator layer 504f and function as coils. The resonance electrode 506a is connected to the outer electrodes 503a and 503c. The resonance electrode 506b is connected to the outer electrode 503c. The resonance electrode 506c is connected to the outer electrodes 503b and 503c. The resonance electrode 506b is magnetically coupled with the resonance electrodes 506a and 506c. In addition to the resonance electrodes 506, the resonators 505a to 505c also include capacitors, but description of such capacitors will be omitted here.
Furthermore, the multilayer dielectric filter 500 includes an adjustment electrode 508. The adjustment electrode 508 is partially superposed with the resonance electrodes 506a and 506c and not superposed with the resonance electrode 506b when viewed in plan from the stacking direction.
In the thus-configured multilayer dielectric filter 500, a high-frequency signal is input from the outer electrode 503a and is output from the outer electrode 503b. The resonance electrode 506a and the resonance electrode 506b are magnetically coupled with each other, and therefore a high-frequency signal input from the outer electrode 503a is transmitted to the resonance electrode 506b via electromagnetic induction. Furthermore, the resonance electrode 506b and the resonance electrode 506c are magnetically coupled with each other, and therefore the high-frequency signal is transmitted to the resonance electrode 506c via electromagnetic induction. Then, the high-frequency signal, which has been transmitted through the resonance electrode 506c, is output from the outer electrode 503b. Here, the resonance electrodes 506a to 506c form the resonators 505a to 505c. Consequently, portions of the high-frequency signal outside the pass frequency band are removed during transmission through the resonance electrodes 506a to 506c. Therefore, a high-frequency signal of the pass frequency band is output from the outer electrode 503b. 
In addition, as illustrated in FIG. 6, the multilayer dielectric filter 500 is provided with the adjustment electrode 508. The low-frequency side of the pass frequency band of the multilayer dielectric filter 500 can be sharply attenuated using this electrode.
However, there is a problem in that, if it is attempted to narrow the pass frequency band, the size of the multilayer dielectric filter 500 is increased. In more detail, in order to narrow the pass frequency band of the multilayer dielectric filter 500, it is sufficient to weaken the magnetic coupling between the resonance electrodes 506a to 506c. However, in order to weaken the magnetic coupling between the resonance electrodes 506a to 506c, it is necessary to increase the distance between the resonance electrodes 506a to 506c. As a result, the multilayer dielectric filter 500 is increased in size.